Anchor devices for well tools and well strings are commonly used to releasably attach well tools and equipment to a well casing or well pipe where the well pipe is a tubular metal member and traverses earth formations. An anchor device can be utilized with a variety of well tool devices, for example with a rod pump, or can be incorporated in a tool such as a bridge plug or packer. Irrespective of the application, the anchor device must be passed in a retracted condition through the bore of the casing or pipe to a location where setting of the anchor is intended. Setting of the anchor device involves extending gripping members radially outward from the device into gripping contact with the bore of the casing or the pipe. Once extended, in releasable anchor devices, the anchor device can be subsequently released and moved to a retracted condition for retrieval. Alternatively, in some releasable devices, if a malfunction prevents retraction from an extended condition, the anchor device can be disabled by actuating a shear means so that the gripping members retract and the anchor device can be retrieved.
In mechanical anchoring devices utilizing slips, the slips are elongated members disposed circumferentially around an annular bowl member an "expander") which has an inclined or wedging surface. The inclined surface on an expander are movable relative to inclined surfaces on the slips and are cooperable to move the slips radially outward where serrated outer surfaces on the slips grip the casing. The wedging action of the inclined surfaces maintains the grip of the slips with respect to a casing.
With a slip operated anchor device there are a number of practical considerations which include, inter alia, the diameter of the expander vis-a-vis the bore through the expander, the angle of the inclined surfaces and the length of the inclined surfaces and the strength of the materials. As a general proposition, it is desirable to maximize the bore size through the expander. Thus, for a given internal diameter of pipe, the effective wall space (the space from the expander bore to the outer diameter of the slips) for the expander and slips is reduced and this, in turn, reduces the available length for inclined surfaces and the angle of inclination required to obtain substantial outward radial travel for the slips. This situation becomes even more pronounced when the well bore or casing has a restricted bore along its length. For example, a well bore can contain a 31/2 inch I.D. bore adjacent to a lower 5 inch I.D. bore. In this instance, the anchor device must traverse through a 31/2 inch bore and expand to a 5 inch bore. Heretofore we have not been aware of any practical mechanical slip anchor devices utilizing slips which can pass through a restricted bore and subsequently be moved radially a substantial distance for gripping engagement with a casing and which can be subsequently retracted and retrieved.